Rheostat

Jonathans

Jonathans

Joined
Jan 9, 2008
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588
I just ordered a new Blacksmith Depot blower for my forge. They warn against using a speed control.
Are there some rheostats that are easier on the motor than others? I would hate to have doled out the cash for a good blower to just burn it up.
 
There are two ways to control the air flow from a forge blower. One is to control the blower speed. This works great for DC blowers, but not so great for many AC blowers.
If using speed controllers on AC blowers, try using the ones made for running a router. However, many smiths just use a light dimmer switch and the blowers seem to do fine.

The other way to control the air flow is with a gate valve and/or a choke plate. The manifold pipe from the blower is where the gate valve is installed. It should be a 2" or3" valve.
A choke plate is a movable "shutter" or plate that closes off part of the blower intake. This is a common way of making an inexpensive air volume control.

Post a link to the exact blower you bought so we can give more specific advice.
 
I have the same blower and between the blower intake vent damper and a gate valve on the blower discharge as Stacy described above, I have good control of the airflow. Here is the burner used with a temporary forge I setup.

15555411717_c168e303f7_o.jpg
 
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Just curious but it seems like gate valve and choke plate would cause the motor to work harder as well. Is the thought that these blowers are more mechanically robust than electrically? I know the forge specific blowers are designed to have good amount of static head.

If running a cheap ceiling fan type blower, seems like changing flow rate via fan speed might take less toll on the blower.

I'm considering a KB variable speed controller. Probably better built despite lower amperage ratings than HF router controller. Blower motors generally seem to only pull a couple amps max.

On a side note, anyone experimented with controlling forge temp with gate valves instead of modulating fan speed?
Having a servo controlled gate valve would be pretty nifty.
 
Boogaloo said:
Just curious but it seems like gate valve and choke plate would cause the motor to work harder as well.
Click to expand...

Restricting air flow does NOT make the motor work harder.
In fact, it's the reverse. If you reduce the amount of air the motor is moving, then you reduce the LOAD against the motor.
It's doing less work.
A shop vac is a blower - sort of - in reverse.
Put your hand over the end of the hose and the motor RPM goes up! The motor is doing less work and has less resistance so it picks up speed.
Remove your hand from the end of the hose and the RPM drops - now there's more resistance against the motor and it's working harder.
Just like closing the choke plate - as you close it the RPM increases as the motor moves less and less air.
 
Karl B. Andersen said:
Restricting air flow does NOT make the motor work harder.
In fact, it's the reverse. If you reduce the amount of air the motor is moving, then you reduce the LOAD against the motor.
It's doing less work.
A shop vac is a blower - sort of - in reverse.
Put your hand over the end of the hose and the motor RPM goes up! The motor is doing less work and has less resistance so it picks up speed.
Remove your hand from the end of the hose and the RPM drops - now there's more resistance against the motor and it's working harder.
Just like closing the choke plate - as you close it the RPM increases as the motor moves less and less air.
Click to expand...

I really don't think this correct. When you constrict inlet/outlet you're introducing back pressure into the system that the blower has to work harder to overcome. Regulating flow geometrically/mechanically is an easier solution and if your blower is burly like the blacksmith depot one the added back pressure is no issue. For a cheap ceiling fan type blower, it's not designed to deal with much static head and adding that extra back pressure may overload the motor.

Here's a link that backs up my thinking: http://www.seai.ie/EnergyMAP/Energy...put_in_place_of_constant_flow_operation1.html


"For fan systems:
-do not apply throttling controls to variable speed or variable pitch fans;
-restricting flow increases system resistance. When fans operate against a higher backpressure, output is reduced, shifting the fan operating point to the left of its performance curve. Maintenance costs may increase when fans do not operate at their best efficiency points;
-vane-type dampers are most effective at 80–100% of full flow. At lower flow rates, inlet vanes become less efficient."
 
No, Carl pretty much has it.
The static pressure of a closed manifold ( restricted outlet) will not make the motor overheat. Yes, the pressure rises in the manifold behind the valve, but the fan blades are spinning in the trapped air ( static) and not bringing in new air. Thus it takes about the same energy as running wide open. The motor spins at full speed all this time, thus the cooling fins pass the max amount of air over the frame and windings....so even though there is no output, the motor is full cooled.

With a choke plate fully closed ( restricted inlet) it is even better, as the fan spins in a partial vacuum, thus having even less work. Again, it is running at full speed and cooling the motor completely.

This is why the choke is adjusted for the minimum opening needed to run the forge at full blast. Then the gate valve is adjusted to lower the blast as desired. Opening and closing the gate valve a tad as needed is all you need to do to trim the forge when running. ( obviously, the gas supply is also adjusted in the process).

I have never done it, but you should be able to verify all the above with a good ammeter.



Going back to my original statements, many people just use a cheap squirrel cage blower and a light dimmer. Sure, it isn't the perfect system, but it works well. Also, a forge isn't usually running at low speed for hours on end...which is how a blower would overheat.


Final comments:
If you really want the max in a forge design, adjustment, and operation ability..... use a DC blower ( 3Ph will also work) and make a two-stage PID controlled burner. This will add $200 at most to the cost of a forge and it can run accurately at almost any desired setting. By having the PID cycle the blower as well as the gas from a pre-set HIGH to a pre-set LOW you can maintain the proper forge atmosphere to avoid excess decarb. Sticking a blade in a forge running at 1500F with a neutral atmosphere and not having to rotate and watch it constantly to avoid burning up the tip or edge is worth $200 to most folks. You can put in three blades and quench them one at a time without fear of ruining the second and third blade.

The whole two-stage PID controlled burner process is covered in the stickys. I designed the burner and gave it to the knife making community for everyone to enjoy. Those who run one are very happy. There are no special skills or tools needed to build one either. If you can make a blown burner, you can make a two stage burner. If you can hook up a few wires, you can make the burner run on PID control. I won't go as far as to say it is idiot proof, because I have met some real idiots in this hobby, but most folks who are ready for a full service forge are more than able to build and use one of my design.
 
Boogaloo said:
-restricting flow increases system resistance. When fans operate against a higher backpressure, output is reduced,
Click to expand...

And? It says output is reduced. It doesn't say anything about it being bad for the motor. Or the motor working harder.
"Output reduced" means it's moving less air.
Put your hand in front of a hair dryer.
The RPM increases. There is less load on the motor.
 
You guys are great. Karl, you brought up a point that should have been obvious, but I never considered that when the rpm went up it was because there was less load, I always interpreted it that the motor was straining. Stacey, I now know how to properly adjust the system I will be working with. I considered the 3 phase motor PID setup but I am not a pro and the forge I'll have already pushed the pocket a bit. Good learning project for the future though!
Thanks very much.
 
Hmm, I guess my thinking was backwards. Seems counterintuitive that a vacuum is not working harder when it is clogged for example.
What makes a blacksmith type blower better than a bathroom fan then (besides general quality and sturdiness)? I thought the higher static head was important?
 
When I have time I'll place an amp meter on it and see how it's affected with different air flows.
The concern that BD had was the motor operating on low voltage rather than overheating
 
Jonathans said:
When I have time I'll place an amp meter on it and see how it's affected with different air flows.
The concern that BD had was the motor operating on low voltage rather than overheating
Click to expand...

Operating at low RPM is why it overheats. Lower voltage creates lower sped and often more heat in the coils. At lower RPM, the heat is not dissipated by the cooling fins of the motor, thus it overheats.
 
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